They go into a lot of detail on how these markets work without a whole lot of discussion as to whether or not this should be happening.
I mean, it isn't surprising in the least. I imagine most USians care less about the effect of the outcome (how many people voting for a major party could tell you much about their own candidate's policies, nevermind the opposition policies) and more on whether their team won or not. It is turning the whole thing into a sports competition, so of course people would gamble on it.
Which I imagine, is exactly the intended result.